CN1121370C - Iridium-catalysed carbonylation process for production of acetic acid - Google Patents

Abstract

The present invention provides a process for the preparation of acetic acid comprising the reaction in the presence of an iridium-containing carbonylation catalyst, a methyl iodide promoter, water, acetic acid and Carbonylation of methanol and/or reactive derivatives thereof with carbon monoxide in a carbonylation reactor of a liquid reaction mixture of methyl acetate, wherein maintaining (i) in the liquid reaction mixture: (a) a concentration of water of less than 5.0% by weight and (g) a concentration of methyl iodide greater than 12% by weight and (ii) a total pressure in the carbonylation reactor of less than 50 bar.

Description

Iridium-catalyzed carbonylation process for the preparation of acetic acid

The invention relates to a method for preparing acetic acid, in particular to a method for preparing acetic acid by carbonylation in the presence of an iridium catalyst and a methyl iodide promoter.

The preparation of carboxylic acids by iridium-catalyzed carbonylation reactions is known and described, for example, in GB-A-1234121, US-A-3772380, DE-A-1767150, EP-A-0616997, EP-A-0618184, Described in EP-A-0618183, EP-A-0657386 and WO-A-96/11179.

GB-A-1234121, US-A-3772380, DE-A-1767150 and WO-A-96/11179, like the present invention, relate to iridium-catalyzed carbonylation processes without the use of promoters.

WO-A-96/11179 discloses inter alia the preparation of alcohols containing (n+1) carbon atoms by carbonylation of alcohols containing (n) carbon atoms in the presence of a catalyst containing at least one iridium compound and at least one halogenated cocatalyst. Atomic carboxylic acid or its corresponding ester method, characterized in that the amount of ester corresponding to carboxylic acid and alcohol in the mixture is maintained at 15-35%, the amount of halogenated co-catalyst is 10-20%, carbon monoxide The partial pressure is 4×10 ⁶ -2×10 ⁷ Pa (40-200 bar), which corresponds to a total pressure of 5×10 ⁶ -2.5×10 ⁷ Pa (50-250 bar).

From, for example, EP-A-0643034, the carbonylation of methanol catalyzed by iridium and promoted by methyl iodide with a promoter selected from ruthenium and osmium in the presence of acetic acid, defined concentrations of water and methyl acetate is known Velocity has beneficial effects.

Nevertheless, there remains a need for an improved iridium-catalyzed carbonylation process in the absence of metallic promoters, such as ruthenium and/or osmium, and/or ionic iodide co-promoters, such as quaternary ammonium and phosphonium iodides.

The present invention provides a process for the preparation of acetic acid comprising the reaction in the presence of an iridium-containing carbonylation catalyst, a methyl iodide promoter, water, acetic acid and Carbonylation of methanol and/or reactive derivatives thereof with carbon monoxide in a carbonylation reactor of a liquid reaction mixture of methyl acetate, characterized in that (i) in the liquid reaction mixture:

(a) the concentration of water is less than 5.0% by weight and

(b) a concentration of methyl iodide greater than 12% by weight and (ii) a total pressure of less than 5 x ¹⁰⁶ Pa (50 bar) in the carbonylation reactor.

The present invention solves the technical problem described above by maintaining a liquid reaction mixture containing defined water and methyl iodide concentrations and a defined total pressure in the carbonylation reactor. This provides several technical advantages.

Increasing the concentration of methyl iodide at relatively low water concentrations has a beneficial effect on the carbonylation rate. Another advantage of using high methyl iodide concentrations at relatively low water concentrations is the reduced rate of formation of one or more of the by-products propionic acid, methane, hydrogen and carbon dioxide.

In the process of the invention methanol and/or its reactive derivatives are carbonylated. Suitable reactive derivatives of methanol include methyl acetate, dimethyl ether and methyl iodide. Mixtures of methanol and its reactive derivatives can be used as reactants in the process of the present invention. Preference is given to using methanol and/or methyl acetate as reactants. At least part of the methanol and/or its reactive derivatives is converted to methyl acetate by reaction with the acetic acid product or solvent in the liquid reaction mixture and thus exists in this form. The concentration of methyl acetate in the liquid reaction mixture of the process of the invention is suitably 1-70% by weight, for example 1-50% by weight, preferably 5-50% by weight, more preferably 10-40% by weight %. If methyl acetate or dimethyl ether is used, water is required to assist the reactants.

The carbon monoxide reactant may be substantially pure or may contain impurities such as carbon dioxide, methane, nitrogen, noble gases, water, and C ₁ -C ₄ paraffins. The presence of hydrogen in the carbon monoxide feed and hydrogen generated in situ by the steam shift reaction is preferably kept as low as possible, since the presence of hydrogen leads to the formation of hydrogenated products. Accordingly, the amount of hydrogen in the carbon monoxide reactant is preferably less than 1 mol%, more preferably less than 0.5 mol%, most preferably less than 0.3 mol% and/or the partial pressure of hydrogen in the carbonylation reactor is preferably less than 1 x ¹⁰ Pa ( 1 bar), more preferably less than 5×10 ⁴ Pa (0.5 bar), most preferably less than 3×10 ⁴ Pa (0.3 bar). The partial pressure of carbon ^monoxide in the carbonylation ^{reactor is} suitably corresponding to a total Press the pressure. The temperature at which the process operates is suitably 100-300°C, preferably 150-220°C.

In the process of the invention, the iridium carbonylation catalyst is preferably present in the liquid reaction mixture in a concentration of 400-5000 ppm measured as iridium, more preferably 500-3000 ppm measured as iridium. In the process of the present invention, as the iridium concentration increases, the rate of the carbonylation reaction increases.

The iridium catalyst in the liquid reaction mixture may contain any iridium-containing compound that is soluble in the liquid reaction mixture. The iridium catalyst may be added to the liquid reaction mixture of the carbonylation reaction in any suitable form dissolved in or converted to dissolved form. Examples of suitable iridium-containing compounds that can be added to the liquid reaction mixture include IrCl ₃ , Irl ₃ , IrBr ₃ , [Ir(CO) ₂ I] ₂ , [Ir(CO) ₂ Cl] ₂ , [Ir(CO) ₂ Br] ₂ , [Ir(CO) ₂ I ₂ ] ^- H ⁺ , [Ir(CO) ₂ Br ₂ ] ^- H ⁺ , [Ir(CO) ₂ I ₄ ] ^- H ⁺ , [Ir(CH ₃ )I ₃ (CO) ₂ ] ^- H ⁺ , Ir ₄ (CO) ₁₂ , IrCl ₃ .3H ₂ O, IrBr ₃ .3H ₂ O, Ir ₄ (CO) ₁₂ , Iridium metal, Ir ₂ O ₃ , IrO ₂ , Ir (acac)(CO) ₂ , Ir(acac) ₃ , iridium acetate, [Ir ₃ O(OAc) ₆ (H ₂ O) ₃ ][OAc] and hexachloroiridate H ₂ [IrCl ₆ ], preferably iridium Chlorine-free complexes, such as acetates, oxalates and acetoacetates, which are soluble in one or more carbonylation reaction components, such as water, alcohol and/or carboxylic acid. Especially preferred is iridium green acetate, which can be used in acetic acid or aqueous acetic acid solution.

The process of the invention is characterized in that the concentration of methyl iodide in the liquid reaction mixture is greater than 12% by weight. The methyl iodide concentration is preferably greater than 14% by weight. The upper limit of the methyl iodide concentration can be up to 20% by weight, usually up to 18%.

Another feature of the method of the invention is that the water concentration is less than 5% by weight.

Water will be generated in situ in the liquid reaction mixture, for example, by the esterification reaction between methanol reactant and acetic acid product. A small amount of water can also be produced by hydrogenation of methanol to produce methane and water. Water can be added to the carbonylation reactor together with the other components of the liquid reaction mixture or separately. Water can be separated from other components in the reaction mixture withdrawn from the reactor and can be recycled in controlled amounts to maintain the desired water concentration in the liquid reaction mixture.

The water concentration in the liquid reaction mixture is less than 5% by weight, preferably less than 4% by weight.

The process of the present invention operates in the substantial absence of metallic promoters, such as ruthenium and osmium, and/or ionic iodide co-promoters, such as quaternary ammonium and phosphonium iodide. For the avoidance of doubt, the term "substantial absence of metal promoters and/or ionic iodide co-promoters" refers to the absence of intentionally added metal promoters and/or ionic iodide co-promoters, as they may be unintentional Existence, for example, through corrosion of the carbonylation reactor, and metals which can act as promoters if intentionally added.

It is especially preferred that the liquid reaction mixture contains about 1-5% by weight water, 14-18% by weight methyl iodide promoter, 14-31% by weight methyl acetate, iridium at a concentration of 400-3000 ppm measured as iridium The catalyst and the remainder of the mixture mainly contain acetic acid, and the preferred reaction conditions are a carbonylation reaction temperature of 185-200°C, a total carbonylation reaction pressure of up to 4 x ¹⁰⁶ Pa (40 bar) and a carbon monoxide partial pressure of 1 ×10 ⁵ -1.2×10 ⁶ Pa (1-12 bar).

Ionic contaminants such as (a) corrosive metals, especially nickel, iron, and chromium, and (b) phosphine or nitrogen-containing compounds or ligands that can be quaternized in situ can be detrimental to the reaction rate by being produced in the liquid reaction mixture Active I ^- have an adverse effect on the reaction, so they should be kept to a minimum in the liquid reaction mixture. Certain corrosion metal contaminants, such as molybdenum, have been found to be less prone to I ⁻ . Corrosion metals that have an adverse effect on the reaction rate can be minimized by using suitable structural corrosion resistant materials. Likewise, contaminants, such as alkali metal iodides, such as lithium iodide, should be kept to a minimum. Corrosion metals and other ionic impurities can be reduced by treating the reaction mixture with a suitable ion exchange resin bed, or preferably a catalyst recycle stream. This method is described in US4007130. Ionic contaminants are preferably kept below concentrations that would yield less than 500 ppm I ⁻ , preferably less than 250 ppm I ⁻ in the liquid reaction mixture.

The process of the invention is preferably operated as a continuous process, but can also be operated as a batch process if desired.

Acetic acid product is recovered from the liquid reaction mixture by venting vapor and/or liquid from the carbonylation reactor and recovering acetic acid from the vented stream. Acetic acid is preferably recovered from the liquid reaction mixture by continuously withdrawing the liquid reaction mixture from the carbonylation reactor, followed by recovery of acetic acid from the withdrawn liquid reaction mixture by one or more flash and/or fractional distillation steps, in the separation steps described above , acetic acid is separated from other components of the liquid reaction mixture, such as iridium catalyst, methyl iodide cocatalyst, methyl acetate, unreacted methanol, water, and acetic acid solvent, which can be recycled to the reactor to keep it in the liquid reaction mixture concentration. To maintain the stability of the iridium catalyst during acetic acid product recovery, water should be maintained at a concentration of at least 0.5% by weight in the iridium carbonylation catalyst-containing product stream for recycle to the carbonylation reactor.

Method of the present invention will be described with reference to following embodiment and accompanying drawing 1 and 2, and accompanying drawing 1 and 2 have represented the function relationship curve of reaction rate to water concentration under different methyl iodide concentrations. In the examples, the following experimental methods are used.

General Description of Experimental Methods Carbonylation Experiments

All experiments used a 300 ml zirconium autoclave equipped with a magnetically driven stirrer with gas dispersing propeller, liquid catalyst injection device and cooling coil. Gas feed to the autoclave is provided by a gas ballast vessel, which feeds gas to maintain the autoclave at a constant pressure. The rate of gas uptake at a location in the reaction experiment is used to calculate the carbonylation rate as per liter of cold degassed reactor per hour for a specific reactor mixture (reactor mixture is measured in cold degassed volume) The number of moles {mol∥l/hr} of reactant consumed by the mixture.

The methyl acetate concentration during the reaction was calculated from the starting mixture assuming one mole of methyl acetate was consumed per mole of carbon monoxide consumed. No organic components were allowed at the top of the autoclave.

For each batch _of carbonylation experiments, catalyst _H2IrCl6 dissolved in a portion of the acetic acid/water liquid reactor feed was added to the liquid injection device. The reactor was then pressure tested with nitrogen, evacuated with a gas sampling system, and flushed several times with carbon monoxide ( ^{3x3x105-1x106 Pa} ) (3x3-10 bar). The remaining liquid components of the reaction mixture were added to the autoclave through the liquid addition port. The autoclave was optionally flushed once with carbon monoxide (1 x about 5 x 10 ⁵ Pa) (1 x about 5 bar). The autoclave was then pressurized with carbon monoxide (typically 6×10 ⁵ Pa (6 bar)) and heated to the reaction temperature, 190° C., with stirring (1500 rpm). The total pressure was raised to about 3 x ¹⁰⁵ Pa (3 bar) below the desired reaction pressure by feeding carbon monoxide from a gas ballast vessel. Once the temperature stabilized (about 15 minutes), the catalyst was injected using carbon monoxide overpressure. The catalyst injection device has >90% efficiency. The reactor pressure was maintained at a constant value ±5×10 ⁴ Pa (±0.5 bar) by gas input from the gas ballast vessel throughout the experiment. Gas uptake into the gas ballast vessel was measured with a data recording device throughout the experiment. The reaction temperature was maintained within ±1°C of the desired reaction temperature by a heating mantle connected to a Eurotherm (trade mark) control system. In addition, excess heat of reaction was removed by cooling coils. Each experiment was performed until the gas intake ceased (ie, gas consumption by the gas ballast vessel was less than 1 x ¹⁰⁴ Pa (0.1 bar) per minute). The gas ballast vessel was then disconnected and the reactor was quenched by using a cooling coil.

_{H2IrCl6 (} 22.2% w/w or 10.6% w/w Ir in water) was supplied by Johnson Matthey. Acetic acid is obtained from the carbonylation of a mixed methanol/methyl acetate feedstock, containing very small amounts of propionic acid and its precursors. Methyl acetate (29,699-6), water (32,007-2) and methyl iodide (1-850-7) were provided by Aldrich.

Example

Experiments 1-9 illustrate the effect of water concentration in % w/w on carbonylation activity using an iridium catalyst at 190 °C and a total reaction pressure of 2.8 x ¹⁰⁶ Pa (28 bar), flowing through the reaction at 30% w/w MeOAc is 16.9% w/w MeI. The feed composition is given in Table 1. Rate data are given in Table 2 at 30%, 25%, 20%, 15%, 10%, 7.5% and 5% w/w MeOAc.

Experiments 1-6(a), 1-5(b), 1-4(c) and 1 and 2(d)(e) and (f) and 1(g) are not methods of the present invention because the water concentration is not Less than 5.0% by weight. They are listed for comparison only.

Experiment AN illustrates the effect of water concentration in % w/w on carbonylation activity using an iridium catalyst at 190 °C and a total reaction pressure of 2.8 x ¹⁰⁶ Pa (28 bar). Flowing through the reaction at 30% w/w MeOAc is 8.4% w/w MeI. The feed composition is given in Table 3. Rate data are given in Table 4 at 30%, 25%, 20%, 15%, 10%, 7.5% and 5% w/w MeOAc.

Experiments A-N are not methods of the invention because the methyl iodide concentration is not greater than 12% by weight. Furthermore, the water concentration was not less than 5.0% by weight in experiments A-J(a), A-F(b), A-E(c), A and B(d), (e) and (f) and A(g). They are listed for comparison only.

The results are illustrated in graph form in Figures 1 and 2 of the accompanying drawings.

Figure 1 illustrates the beneficial effect of increasing the MeI concentration from 8.4% w/w to 16.9% w/w at 30% w/w MeOAc and a water concentration of less than 5% w/w and a total pressure of 2.8 x ¹⁰⁶ Pa (28 bar) .

Figure 2 illustrates the beneficial effect of increasing the MeI concentration from 8.0% w/w to 16.0% w/w at 15% w/w MeOAc and water concentrations less than 5% w/w and a total pressure of 2.8 x ¹⁰⁶ Pa (28 bar) .

According to Figures 1 and 2, it is especially beneficial to increase the concentration of methyl iodide to greater than 12% w/w under total pressure and carbon monoxide partial pressure. The ester concentration reaction rate decreased with decreasing water concentration. Table 1 The feed composition of iridium catalyzed reaction in 300ml zirconium batch autoclave Experimental batch number MeOAc/g AcOH/g MeI/g water/g H ₂ IrCl ₆ /a 1 6842 6243 6854 7025 6966 6147 6838 6869 682 60.02 34.07 27.03 28.26 0.64369.98 42.26 27.03 20.22 0.64360.00 46.34 27.04 16.00 0.64260.07 47.48 27.04 14.88 0.64460.00 48.74 27.05 13.56 0.64160.02 50.43 27.01 11.90 0.64260.01 52.86 27.03 9.47 0.64260.00 54.40 27.03 7.96 0.64160.03 55.92 27.03 6.40 0.641

a) Table 2 Iridium-catalyzed reaction rate in 300ml autoclave in terms of pure H ₂ IrCl ₆ weight; Effect of water concentration rate under various MeOAc concentrations when MeI concentration is about 16% w/w ^* experiment (a) (b) (c) (d) (e) Water/% Rate/w/w mol/hr@30%MeOAc Water/% Velocity/w/w mol/hr@25%MeOAc Water/% Velocity/w/w mol/hr@20%MeOAc Water/% Velocity/w/w mol/hr@15%MeOAc Water/% Velocity/w/w mol/hr@10%MeOAc 1 16.1 4.9 14.7 5.5 13.4 6.1 12.0 6.2 10.6 5.3 2 10.9 11.8 9.7 13.1 8.4 13.7 7.1 11.6 5.8 8.6 3 8.2 20.1 7.0 18.6 5.7 16.6 4.5 14.3 3.3 10.6 4 7.5 20.4 6.3 18.3 5.1 16.1 3.8 13.6 2.6 10.3 5 6.6 22.4 5.4 20.4 4.2 17.7 3.0 14.7 1.8 11.5 6 5.6 23.2 4.4 21.2 3.2 18.8 2.0 15.8 0.8 10.1 7 4.0 27.4 2.8 22.6 1.7 17.0 0.5 9.4 - - 8 3.0 26.9 1.8 20.2 0.7 12.4 - - - - 9 2.0 21.8 0.9 10.9 - - - - - -

Table 2 continued (f) (g) Water/rate/mol/hr@w/w 7.5% MeOAc Water/% Rate/mol/hr@w/w 50% MeOAc 1 9.9 4.5 9.3 3.2 2 5.2 6.4 4.5 NA 3 2.7 8.3 2.1 6.4 4 2.0 8.6 1.4 6.5 5 1.2 9.2 0.6 5.9 6 0.2 5.1 - - ^* The whole reaction is at 2.8× ¹⁰⁶ Pa (28 bar) total pressure, 190°C stirring rate 1500rpm, about 16.9% MeI, 30% MeOAc. About 16.0% MeI, 15% MeOAc. According to about 4 moles of formazan per mole of iridium Base iodide makes [Ir(Co) ₂ I ₄ ] Slight down-regulation of MeI concentration NA - not available, reaction ends too early. Table 3 is composed of raw materials in a 300ml zirconium batch autoclave. Experimental batch number MeOAc/ _g AcOH/g MeI/g water/g _H2IrCl6 /g ^* A 630B 609C 641D 653E 675F 731G 598H 615I 621J 634K 640L 643M 763N 642 60.07 47.13 13.96 28.30 0.63959.99 55.32 13.97 20.11 0.64060.01 59.40 13.96 16.06 0.64160.02 59.52 13.97 16.00 0.64360.02 59.42 13.96 15.99 0.64360.00 61.89 13.97 13.71 0.64359.99 63.54 13.97 11.94 0.64160.02 63.51 13.96 11.96 0.64059.99 63.49 13.96 11.96 0.64060.05 63.49 13.96 11.98 0.64960.03 65.95 13.97 9.51 0.64460.01 66.15 13.96 9.52 0.64660.00 67.51 13.96 7.91 0.634.02 68.993.96 6.46426422

^* Represented by weight of pure H ₂ IrCl ₆ Table 4 Iridium catalyzed reaction rate in 300mol batch autoclave; at about 8% w/w MeI, at

Effect of water concentration on rate at various MeOAc concentrations. ^* experiment (a) (b) (c) (d) (e) Water/% Rate/w/w mol/hr@30%MeOAc Water/% Velocity/w/w mol/hr@25%MeOAc Water/% Velocity/w/w mol/hr@20%MeOAc Water/% Velocity/w/w mol/hr@15%MeOAc Water/% Velocity/w/w mol/hr@10%MeOAc A 16.1 7.1 14.8 7.5 13.4 8.0 12.0 7.7 10.7 6.9 B 10.9 14.9 9.6 14.6 83 13.5 7.0 11.2 5.8 7.1 C 8.2 17.7 7.0 15.7 5.8 13.4 4.5 10.5 3.3 7.5 D. 8.2 18.2 7.0 17.0 5.7 15.1 4.5 11.7 3.3 8.4 E. 8.2 18.6 7.0 16.7 5.7 14.4 4.5 11.8 3.3 8.7 f 6.7 19.6 5.5 17.4 4.3 14.9 3.1 12.0 1.9 8.7 G 5.6 20.4 4.4 17.0 3.2 13.8 2.0 9.5 0.8 4.9 h 5.6 19.7 4.4 17.6 3.2 14.3 2.0 9.9 0.9 5.0 I 5.6 20.5 4.4 17.4 3.2 14.0 2.0 10.5 0.9 5.5 J 5.6 21.1 4.4 17.9 3.2 14.8 2.0 11.1 0.9 6.0 K 4.0 19.7 2.9 15.5 1.7 10.8 0.5 N/A - - L 4.0 22.0 2.9 16.1 1.7 9.9 0.6 4.4 - - m 3.0 20.4 1.9 14.1 0.7 6.9 - - - - N 2.1 12.1 0.9 5.9 - - - - - - Table 4 continued (f) (g) Water/rate/mol/hr@w/w 7.5% MeOAc Water/% Rate/mol/hr@w/w 50% MeOAc A 10.0 6.0 9.3 4.8 B 5.1 5.4 4.5 3.9 C 2.7 6.0 2.1 4.5 D. 2.7 6.9 2.0 5.4 E. 2.7 6.9 2.0 5.1 f 1.3 6.5 0.7 3.4 ^* The whole reaction is at 2.8 x ¹⁰⁶ Pa (28 bar) total pressure, 190°C stirring rate 1500rpm, about 8.4% MeI, 30% MeOAc. About 8.0% MeI, 15% MeOAc. According to about 4 moles of formazan per mole of iridium Base iodide makes [Ir(Co) ₂ I ₄ ] Slight down-regulation of MeI concentration NA - not available, reaction ends too early. N/A = reaction ended too early, no rate could be measured at that point.

Claims (16)

1. A process for the preparation of acetic acid, which comprises the process of containing iridium-containing carbonylation catalyst, methyl iodide promoter, water, acetic acid and methyl acetate under the absence of metal promoter and/or ion iodide auxiliary promoter substantially. Carbonylation of methanol and/or reactive derivatives thereof with carbon monoxide in a carbonylation reactor of a liquid reaction mixture of esters, characterized in that (i) is kept in the liquid reaction mixture: (a) the concentration of water is less than 5.0% by weight, and (b) a concentration of methyl iodide greater than 12% by weight, and (ii) a total pressure in the carbonylation reactor of less than 5.0 x ¹⁰⁶ Pa (50 bar). 2. A process according to claim 1, wherein methanol and/or methyl acetate is carbonylated. 3. The process according to claim 1, wherein the concentration of methyl acetate in the liquid reaction mixture is 1-50% by weight. 4. The process according to claim 3, wherein the concentration of methyl acetate in the liquid reaction mixture is 10-40% by weight. 5. A process according to any preceding claim, wherein the concentration of methyl iodide in the liquid reaction mixture is greater than 14% by weight. 6. The method according to claim 5, wherein the upper limit of the concentration of methyl iodide is 20% by weight. 7. A process according to any one of claims 1-4, wherein the concentration of water in the liquid reaction mixture is less than 4% by weight. 8. A process according to any one of claims 1-4, wherein the concentration of iridium catalyst in the liquid reaction mixture is 400-5000 ppm measured as iridium. 9. A process according to claim 8, wherein the concentration of iridium catalyst is 500-3000 ppm measured as iridium. 10. A process according to any one of claims 1-4, wherein the total pressure in the carbonylation reactor is less than 4.0 x ¹⁰⁶ Pa (40 bar). 11. The method of claim 10, wherein the total pressure is less than 3.0 x ¹⁰⁶ Pa (30 bar). 12. A process according to any one of claims 1-4, wherein the operating temperature is 150-220°C. 13. A process according to any one of claims 1-4, wherein the amount of hydrogen in the carbon monoxide reactant is less than 0.3 mol%. 14. A process according to any one of claims 1-4, wherein the partial pressure of hydrogen in the carbonylation reactor is less than 3 x ¹⁰⁴ Pa (0.3 bar). 15. The method according to claim 1, wherein the liquid reaction mixture contains: 1-5% water by weight, 14-18% by weight methyl iodide, 14-31% methyl acetate by weight, an iridium catalyst at a concentration of 400-3000 ppm measured as iridium, and The remainder mainly contains acetic acid, and the reaction conditions are such that the carbonylation reaction temperature is 185-200° C., the total pressure of the carbonylation reaction is up to 4.0×10 ⁶ Pa (40 bar) and the carbon monoxide partial pressure is 1×10 ⁵ -1.2×10 ⁶ Pa (1-12 bar). 16. A process according to any one of claims 1-4 and 15, which is operated continuously.